It is known that deposition of metallic tungsten on semiconductor materials in the fabrication of VLSI integrated circuitry is useful. For this application, the tungsten metal deposited should be, to the highest degree possible, devoid of alpha-particle emitting impurities such uranium and thorium which can cause soft error generation in memory circuits. Impurities such as sodium also must be eliminated from the tungsten used for this purpose to avoid diffusion thereof from the tungsten metal layer into active regions of the circuit, an event which could destroy or degrade circuit performance.
Industrial production of high purity tungsten traces back to the development of tungsten wire for use in incandescent lamp filaments since it was found that certain impurities, such as iron, rendered tungsten billets brittle and unworkable. The development of the technology is recounted in Smithells book Tungsten, A. Treatise On Its Metallurgy, Properties And Applications published by Chapman & Hall Ltd., London, 1952.
In providing a tungsten product for use in semiconductors it is found that impurity levels in the tungsten must be far lower, i.e., orders of magnitude lower, than that required merely to make tungsten ductile. The present invention is directed to means and methods of providing a tungsten product which may be employed in treating semiconductor materials without producing adverse or destructive effects in circuits produced using such a tungsten product.
The invention is particularly directed toward the production of tungsten hexafluoride of ultra-high purity which may be employed to deposit metallic tungsten on semiconductor materials without producing the aforementioned deleterious effects on the resulting circuits.
Deposition of tungsten from tungsten hexafluoride offers practical advantages as compared to deposition of tungsten by the sputtering method as has been practiced in the semiconductor industry.